1. Field of the Invention
This invention relates in general to earth boring drill bits, and in particular to nozzle placement in large diameter earth boring bits.
2. Description of the Prior Art
This invention concerns large diameter bits, typically ranging in diameter from four feet to fifteen feet, that are used for drilling shafts for mining and other purposes. These bits have a generally circular cutter support plate with a number of rotatable cutters mounted to the plate bottom. The bit is rotated by a string of drill pipe. Cuttings are cleaned from the borehole by circulating fluid.
In one prior art method, a mixture of air and liquid is pumped down the annular passage between the inner and outer drill pipe of a string of dual concentric pipe. In a diffuser the air separates from the water and enters the inner passage through a set of air nozzles initiating the so called reverse air lift circulation. In this circulation method, liquid from the borehole annulus flows around the sides of the bit and then across the bottom to return up the inner passage. The water level in the borehole annulus is kept at a predetermined level by pumping a sufficient amount of liquid in the hole. The liquid which is pumped with the air down the annular passage flows to the bottom of the bit and is discharged through a plurality of nozzles mounted to the bottom of the cutter support. There it combines with the flow from the borehole annulus to return up the inner passage.
In this prior art method, the nozzle discharge is used primarily to create turbulence for suspending the cuttings. The suspended cuttings are not swept toward the intake by the nozzles, rather are swept toward the intake by the major flow of liquid from the borehole annulus. Consequently, the nozzles are oriented in various directions and positioned at various points about the cutter support plate to discharge against the borehole for maintaining the particles in suspension.
Other prior art methods of cleaning the borehole of cuttings are shown in U.S. Pat. Nos. 3,297,100 and 3,360,061. U.S. Pat. No. 3,297,100 discloses a large diameter bit having nozzles directed vertically downward for discharging liquid to suspend the cuttings in the turbulence. U.S. Pat. No. 3,360,061 teaches the circulation of gas through nozzles to sweep cuttings into a scoop for returning up the inner passage.
The prior art devices have need for improvement. It would be beneficial for example to increase the efficiency of cuttings removal to reduce wear on the cutters and increase penetration rates.